Resiliently deformable bellows are used as sealing boots around flexible couplings, as flexible couplings themselves, as collapsible containers, etc. A particular application of interest for a bellows is as a replacement for a piston, return spring, and cylinder in pump devices, particularly lotion pumps and trigger sprayers.
Pump devices require pressurization of a fluid in order to dispense the fluid. Pistons and cylinders have historically been used to generate fluid pressure. In a trigger sprayer which is used predominantly for low viscosity fluids, for example, a piston and cylinder arrangement provide either high friction and good fluid sealing, or low friction and a certain amount of bypass leakage. Neither of these combinations is desirable. Piston and cylinders are also known for binding during axial movement, which causes erratic actuator motion.
A cylindrical resilient bellows serves as both a pumping chamber and a return spring. The bellows changes volume by virtue of axial deformation. A bellows has a series of interconnected pleats which have walls that act as conical ring springs. The summation of conical ring bending resistance's for all of the individual pleats determines the axial resilience of the bellows. The primary advantage of a bellows is that it can provide a lower actuation force than a piston, return spring, and cylinder combination. A bellows resistance to axial deflection is less than the sliding friction between piston and cylinder and the force of a piston return spring. Also, a bellows is a single part, and therefore it has no assembly and critical fit considerations. Because there are no sliding parts, there is no dynamic sealing required for a bellows.
A bellows is statically sealed to the moving parts of a pump device. This is typically achieved by connecting two open ends of a bellows to rigid actuator parts by snap-fitting or thermal or adhesive bonding. The actuator parts may contain one-way valving so that when the actuator is released, the bellows pumping chamber expands due to bellows resilience, thereby drawing in fluid through an inlet valve. When the actuator is actuated, the bellows pumping chamber contracts, pressurizing the fluid as the inlet valve closes while an exit valve opens to discharge the fluid.
Manufacturing a bellows with both ends open is not without difficulties. Such bellows may be made by blow molding or by injection molding. Blow molding is a simpler process, but it results in less precise wall thickness of the conical ring portions of the pleats. Resistance to bellows pleat deflection is a function of conical ring thickness cubed. Therefore, trigger sprayer bellows are made by injection molding so that their axial deflection resistance is more controllable. Also, injection molding permits the addition of other functional elements, such as valves and valve springs, atomizer elements, etc., molded integrally with the bellows.
Injection molding of bellows is a well known process. The trick, of course, when injection molding any part with radial undulations or undercuts, is stripping the part from the mold elements. Typically, injection molded bellows are hand stripped or pushed off a mold core pin. Such stripping of a still warm part often results in permanent damage to the bellows from abrasion with the mold. Alternatively, bellows have been molded with one end closed so that air can be used to inflate the bellows to blow it off the core pin. However, the resulting bellows must thereafter undergo a post processing operation to remove the closed end, and additional scrap results.
Arima et al. U.S. Pat. No. 5,006,376, issued Apr. 9, 1991, discloses an open ended conical bellows shape which has specific proportions and is made of specific material in order that it may be stripped from an injection mold. The extreme limitations on proportions and materials exemplifies the difficulty of stripping a bellows from a mold without damaging the bellows.
What has been missing is a method for injection molding a bellows having both ends open as molded, which uses a compressed gas to expand the bellows while it is pulled off the core pin of the mold. Such a method, which is automated to have low cycle time and which requires no external mechanisms, is an object of the present invention.